1. Field of the Invention
This invention relates to improved resin based semi-conductive compositions. More particularly this invention relates to resin based semiconductive compositions which exhibit controlled strippability when applied as an external semiconductive layer on cross-linked ethylene polymer based insulation which is employed in power cables.
2. Description of the Prior Art
Heretofore, power cables which are insulated with cross-linked ethylene polymer based insulation compositions have been further coated with an extruded semiconductive layer of a resin based composition. The semiconductive layer is applied to the insulation layer so as to closely adhere thereto and provide a gas and moisture tight seal between the two layers. The resin based semiconductive compositions which have been heretofore used for this purpose include cross-linkable compositions which are based on ethylene-ethyl acrylate or ethylene-vinyl acetate copolymers, and which also contain conductive carbon black and organic peroxides as crosslinking agents. When the thus coated power cables are used in the field, however, portions of the external semiconductive layer have to be completely removed, relatively quickly, from the cable for certain purposes. The need for thus quickly removing portions of the semiconductive layer from the insulated power cable may arise, for example, in making connections between two ends of such cables, and also when joining the cables to terminals. For such purposes, therefore, it is highly desirable that the semiconductive layer be readily strippable from the insulation layer to which it adheres.
This requirement, that the semiconductive layer be readily strippable from the insulation layer, has not been met by many conventionally used resin based semiconductive compositions. Many of the conventional semiconductive compositions adhere too strongly to the insulation layer, thereby rendering it impossible to readily strip or peel the semiconductive layer from the insulation layer. Where the semiconductive layer adheres too strongly to the insulation layer it may require too long a time, for practical purposes, to adequately remove the desired amount of semiconductive material from the insulation layer. Also, in the process of removing a strongly adhering semiconductive layer, portions of the underlying insulation layer may be unintentionally pulled off too, thus damaging the insulation layer. It is highly desirable, therefore, for commercial purposes, to provide resin based semiconductive compositions which can be used as external, adhering, coatings for power cable insulated with cross-linked ethylene polymer based insulation compositions, and which can be readily stripped away from the insulation layer when necessary.